The fractionation of a mixture of several gaseous components is usually performed either by direct treatment of the mixture in gaseous form, for example by using techniques of adsorption on a solid, absorption in a liquid or permeation through membranes, or by liquefaction, by cooling and/or compressing the whole or a part of the gaseous mixture, often followed with the treatment of the resultant liquid by distillation or solvent extraction.
Any separation process consumes power; consequently, the power consumption of a fractionation process is an essential feature in the selection of such a process. In processes directly treating the mixture in gaseous form, i.e. by adsorption on a solid, absorption in a liquid or permeation in gaseous phase, the power consumption is relatively moderate but the separated fluid is a gas generally obtained at a relatively low pressure, which, depending on its subsequent use, may need to be recompressed and/or liquefied, thus requiring a high power consumption. In the case of a separation by distillation or extraction, the liquefaction takes place before the separation; it is often necessary, depending on the pressure in the rectification column, to cool the condenser of the distillation column down to a low temperature, which requires a further power consumption; it is notable, on the other hand, that the extraction by solvent is almost always followed with a distillation, for recycling the solvent. This second type of separation process, treating a liquid phase, provides a separated fluid in liquid phase but at the cost of a high power consumption.
In the past several years, numerous surveys have been conducted on the extraction by means of supercritical fluids. This technique takes advantage of the large selectivity variation of a fluid in response to temperature near its critical point. These processes offer advantages resulting from the great diffusion power and the low density and viscosity of the supercritical solvent; their power consumption is relatively low but they have the disadvantage of requiring very high pressures (usually from 5 to 30 MPa).
When separating a gaseous mixture by absorption in a liquid, a portion of the mixture components is dissolved in said absorption liquid and hence can be considered as being in liquid state; it is then re-extracted by increasing the temperature and/or decreasing the pressure and restoring the gaseous phase. As a matter of fact, it is a general rule that the absorption of a gas in a liquid is increasingly more substantial as the temperature is lower and the pressure higher.
The invention is based on a concept different from the process that is mentioned above, specifically on the concept of demixing, in two separate liquid phases, of a solution of a gas in a liquid, this demixing occurring, in some cases, as a result of a temperature decrease.